Pervaporation is a separation technology that is beginning to achieve commercial success. The pervaporation process itself has been known since the 1950s, and is described, for example, in U.S. Pat. Nos. 2,913,507 and 2,953,502 to Binning et al. Organic-selective pervaporation is described in a general way in U.S. Pat. No. 4,218,312 to Perry. Despite the theoretical knowledge embodied in these patents, many years elapsed before commercially viable pervaporation systems could be contemplated, because the technology to make high performance membranes and modules had not been developed. This technology has begun to emerge in the last few years.
The design of a pervaporation system depends on the nature of the stream to be treated, the desired compositions of the permeate and residue streams and the properties of the available membranes. One pass through a single bank of pervaporation modules may be adequate to achieve a sufficiently concentrated permeate and a sufficiently depleted residue, so long as the starting concentration is within a particular range, and so long as the membrane is intrinsically capable of that level of separation. It is frequently the case, however, that either the permeate stream or the residue stream emerging from the module bank, or both, will need to pass through a second module bank before reaching the target concentration. Often, recycling of streams between module banks will be necessary. It is generally necessary, therefore, to make a custom-designed system for each application. For large industrial plants with feed streams of a constant composition and steady, high-volume flow rates, this is acceptable. To date, however, the benefits of pervaporation as an efficient separation technology have not been readily available to potential users who do not fit this description, particularly small industries, those with streams whose flow rate and/or composition is variable, or those whose stream flow rates are low. Such potential users may not have the resources of large plants to pool, adjust or pretreat feed streams to bring them within tight starting parameters for treatment. They may also encounter problems using other separations and waste-treatment technologies, such as distillation, carbon adsorption, incineration, air or steam stripping, because of the size, complexity, inflexibility or expense of the equipment, and recently, because of emissions control legislation. There exists, therefore, a need for separation equipment that offers flexibility from a standard design and that is within the economic and technical resources of relatively small companies.